Method, device and system for forwarding multicast packets

ABSTRACT

In the field of switching networks, a method, a device and a system for forwarding multicast packets are disclosed, which significantly reduce the requirement on the storage capacity of the switching network, thus implementing forwarding of a large number of multicast packets. The method includes: receiving a multicast packet, and a first forwarding identifier and a second forwarding identifier of the multicast packet; obtaining a destination line card corresponding to the multicast packet according to the first forwarding identifier; and forwarding the multicast packet and the second forwarding identifier to the destination line card. The method, device and system for forwarding multicast packets are applicable to situations where it is required to forward multicast packets in a single-level switching network or a multi-level switching network.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Chinese Patent Application No.201010104815.6, filed on Jan. 29, 2010, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of switching networks, and inparticular, to a method, a device and a system for forwarding multicastpackets.

BACKGROUND OF THE INVENTION

A switching network in the prior art can provide unicast services andmulticast services. The unicast services are point-to-point services,for example, an Internet access service is a common unicast service. Themulticast services are point-to-multipoint services, and commonmulticast services include an Internet Protocol Television (IPTV)service and a Layer 2 multipoint Virtual Private Network (VPN) service,such as a Virtual Private Lan Service (VPLS) service. As the multicastservices gradually rise, it becomes a hot research spot.

Take the IPTV service as an example. When multicast packets areforwarded, one channel represents one multicast data stream and onemulticast data stream is indicated by one Multicast Group Identifier(MID). Viewers who watch the channels (may be called “multicast groupmembers”) may be different, the viewers are connected to differentphysical ports of a network device, and the network device forwards indistinction each multicast data stream in order to avoid wastingbandwidth. For example, if the physical port is the one indicated by theMID, a multicast packet is sent to the physical port; otherwise, nomulticast packet is sent to the physical port.

The prior art provides a method for implementing multicast packetforwarding by a two-stage replication technology using the same MID.After receiving a multicast packet, a line card obtains the MID of themulticast packet by looking up a table, and then the line card sends themulticast packet together with the MID to a switch fabric card. Theswitch fabric card performs first stage replication, in which the switchfabric card uses the MID as an index to look up a multicast replicationtable saved therein, obtains line cards corresponding to the multicastpacket, and sends the multicast packet and the MID to the line cards.The line cards perform second stage replication, in which the line cardsuse the MID as an index to look up a multicast replication table savedtherein, obtain physical ports corresponding to the multicast packet,and send the multicast packet to each physical port indicated by theMID.

In the implementation of the present invention, the inventor finds thatthe prior art at least has the following problems. In the currentmulticast services, the required number of the multicast groups isincreasingly growing, for example, up to 1 M (million) multicast groups.At this time, the capacity of a multicast replication table saved by aswitch fabric card also becomes increasingly larger. For example, in asystem with 1 M multicast groups and 64 line cards, the requiredcapacity of the multicast replication table is 1 M*64 bit=64 Mbit.However, the storage capacity of the switch fabric card is merely tensof K, which can hardly meet the requirement, thus failing to implementmulticast packet forwarding.

SUMMARY OF THE INVENTION

In order to solve the problems in the prior art, the present inventionis directed to a method, a device and a system for forwarding multicastpackets.

In order to achieve the above objectives, the present invention adoptsthe following technical solutions.

A method for forwarding multicast packets is provided, which includes:

receiving a multicast packet, and a first forwarding identifier and asecond forwarding identifier of the multicast packet;

obtaining a destination line card corresponding to the multicast packetaccording to the first forwarding identifier; and

forwarding the multicast packet and the second forwarding identifier tothe destination line card, in order that the destination line cardobtains a port corresponding to the multicast packet according to thesecond forwarding identifier.

A network device is provided, which includes:

a receiving unit, configured to receive a multicast packet, and a firstforwarding identifier and a second forwarding identifier of themulticast packet;

a line card obtaining unit, configured to obtain a destination line cardcorresponding to the multicast packet according to the first forwardingidentifier received by the receiving unit; and

a forwarding unit, configured to forward the multicast packet and thesecond forwarding identifier to the destination line card, in order thatthe destination line card obtains a port corresponding to the multicastpacket according to the second forwarding identifier.

A switching system is further provided. The system includes at least oneline card which includes destination line cards. The system furtherincludes at least one network device.

The network device is configured to receive a multicast packet, and afirst forwarding identifier and a second forwarding identifier of themulticast packet, obtain a destination line card corresponding to themulticast packet according to the first forwarding identifier, andforward the multicast packet and the second forwarding identifier to thedestination line card, in order that the destination line card obtains aport corresponding to the multicast packet according to the secondforwarding identifier. The destination line card is configured to obtainthe port corresponding to the multicast packet according to the secondforwarding identifier, and forward the multicast packet to the port.

The technical solutions of the present invention use two identifiers toimplement multicast packet forwarding. The first forwarding identifieris configured to obtain the destination line card corresponding to themulticast packet, and the second forwarding identifier is configured toobtain the port corresponding to the multicast packet. By controllingthe number of the first forwarding identifiers, the capacity of amulticast replication table saved by a switching network can be reduced.The technical solutions of the present invention remove the redundantdata stored repeatedly in the switching network, and reduce therequirement on the storage capacity of the switching network, thusimplementing forwarding of a large number of multicast packets.

DETAILED DESCRIPTION OF THE DRAWINGS

To illustrate the technical solution according to the embodiments of thepresent invention or in the prior art more clearly, the accompanyingdrawings for describing the embodiments or the prior art are givenbriefly below. Apparently, the accompanying drawings in the followingdescription are only some embodiments of the present invention, andpersons of ordinary skill in the art can derive other drawings from theaccompanying drawings without creative efforts.

FIG. 1 is a schematic flow chart of a method for forwarding multicastpackets according to an embodiment of the present invention;

FIG. 2 is a schematic structural view of a router for implementingmulticast packet forwarding according to another embodiment of thepresent invention;

FIG. 3 is a schematic view of an implementation of a line card accordingto another embodiment of the present invention;

FIG. 4 is a schematic structural view of a line card having sub-linecards according to another embodiment of the present invention;

FIG. 5 is a schematic structural view of a network device according toyet another embodiment of the present invention;

FIG. 6 is a schematic structural view of another network deviceaccording to yet another embodiment of the present invention;

FIG. 7 is a schematic structural view of yet another network deviceaccording to yet another embodiment of the present invention; and

FIG. 8 is a schematic structural view of a switching system according toyet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The technical solutions of the present invention are elaborated belowwith reference to accompanying drawings. Evidently, the embodimentsdescribed below are for the exemplary purpose only, without covering allembodiments of the present invention. Persons of ordinary skill in theart can derive other embodiments from the embodiments given hereinwithout making any creative effort, and all such embodiments are coveredin the protection scope of the present invention.

According to an embodiment of the present invention, a method forforwarding multicast packets is provided. Referring to FIG. 1, themethod includes:

Step 11: receiving a multicast packet, and a first forwarding identifierand a second forwarding identifier of the multicast packet;

Step 12: obtaining a destination line card corresponding to themulticast packet according to the first forwarding identifier; and

Step 13: forwarding the multicast packet and the second forwardingidentifier to the destination line card, in order that the destinationline card obtains a port corresponding to the multicast packet accordingto the second forwarding identifier.

In order to describe clearly the technical solutions of the presentinvention, in the embodiments of the present invention, the terms“first” and “second” are used to distinguish the same items or similaritems with basically the same function and effect.

The Steps 11 to 13 can be implemented by a switching network. After theStep 13, an embodiment of the present invention further includes:forwarding, by the destination line card, the multicast packet to thecorresponding port by using the received second forwarding identifier,in which, the port may be a physical port or logical port. The firstforwarding identifier is a newly added identifier, and an existing MIDmay be used as the second forwarding identifier.

The number of the first forwarding identifiers is controlled accordingto the capacity of the switching network, and the first forwardingidentifier may be set in various manners. In an embodiment, when thestorage capacity of the switching network is very limited, all themulticast packets with the same destination line card have the samefirst forwarding identifier, or when the storage capacity of theswitching network is abundant, all the multicast packets with the samedestination line card are divided into several groups, and the samefirst forwarding identifier is set for the multicast packets in eachgroup.

The technical solution according to the embodiment of the presentinvention uses two identifiers to implement multicast packet forwarding.The first forwarding identifier is configured to obtain the destinationline card corresponding to the multicast packet, and the secondforwarding identifier is configured to obtain the port corresponding tothe multicast packet. By controlling the number of the first forwardingidentifiers, the capacity of a multicast replication table saved by theswitching network can be significantly reduced. The technical solutionaccording to the embodiment of the present invention significantlyreduces the requirement on the storage capacity of the switchingnetwork, thus implementing forwarding of a large number of multicastpackets.

Another embodiment of the present invention provides a method forforwarding multicast packets, which is described in detail in thefollowing.

The embodiment of the present invention is described by taking ascenario where a switching network implements multicast packetforwarding through switch fabric cards as an example, but the presentinvention is not limited thereto, and other functional modules or otherchips integrated with switch fabric cards may be used to implementmulticast packet forwarding. In the embodiment of the present invention,the replication function of switch fabric cards and the replicationfunction of line cards are decoupled, and compared with the processingmethod in the prior art that the same identifier is used to controltwo-stage replication, different identifiers are used to control theswitch fabric cards and the line cards respectively, the number of theidentifiers of the switch fabric cards is small (for example, merelytens of K), and the number of the identifiers of the line cards is large(for example, 1 M), thus reducing the requirement on the capacity of theswitch fabric cards.

Because data on the line cards is stored on an off-chip Random AccessMemory (RAM), such as a Dynamic Random Access Memory (DRAM) or StaticRandom Access Memory (SRAM), it is no doubt that identifier informationrequired by 1 M multicast packets can be stored. The present inventiondose not adopt the processing method of setting an off-chip RAM for theswitch fabric cards, because this solution leads to a high cost,increases the volume occupied by the board and reduces the deviceintegration.

Multicast packet forwarding may be implemented through a network devicewith internal switch fabric cards, such as a router or an Ethernetswitch in the embodiment of the present invention. FIG. 2 is a schematicstructural view of a router according to an embodiment of the presentinvention. Referring to FIG. 2, the router includes several line cardsand switch fabric cards, in which the switch fabric cards interconnectthe line cards, and may be implemented through an independent board orby integrating on a main control chip of the router.

One line card or more line cards may be set on one board. Referring toFIG. 3, line card A and line card B are set on one board. One line cardprovides one or more physical ports, or may provide one or more logicalports, and one physical port may contain a plurality of logical ports.The line cards receive or send data packets through physical portsand/or logical ports. The processing capability of the line cards isusually 10 Gbps or 40 Gbps, and common physical ports provided by theline cards include Gigabit Ethernet (GE) interface, LOGE, OC48 POS(Optical Carrier-48 Packet Over SONET), and OC192 POS.

When multicast packets are forwarded, the following steps are performed.

In Step S1, a line card at the input end sends a multicast packet andtwo forwarding identifiers to a switch fabric card.

It can be known through in-depth analysis that, in the process oftwo-stage replication during multicast packet forwarding, the switchingnetwork needs to know the destination line card corresponding to allmulticast packet members of one multicast group, and the destinationline card needs to know a set of physical ports and/or logical portscorresponding to all multicast packet members of one multicast group.Because the number of the line cards is one or two orders of magnitudesmaller than the number of the ports, the total number of the set of thedestination line cards is much smaller than the total number of the setof the ports.

Therefore, corresponding to each multicast packet, two differentidentifiers are set for forwarding, for example, a first forwardingidentifier expressed as MID_F and a second forwarding identifierexpressed as MID_S. MID_F indicates the set of the destination linecards, MID_S indicates the set of the physical ports or logical ports,and the number of MID_F is much smaller than the number of MID_S. Forexample, as for a system with 1 M multicast packets and 64 line cards,if each multicast packet has one or more users on the 64 line cards(that is to say, each multicast packet has one or more ports on the 64line cards), that is, the multicast packets have the same destinationline cards, and one or more users on the line cards are distributedrandomly in the 48 GE interfaces. At this time, the number of MID_F isone (indicating broadcast to 64 line cards), and the number of MID_S isat most 1 M (if every port is different, the number of MID_S is themost, 1 M; if not every point is different, the number of MID_S issmaller than 1 M).

After the line card at the input end receives the multicast packet,relevant data is looked up to obtain the first forwarding identifier andthe second forwarding identifier, in which, the relevant data contains acorresponding relation between multicast packets, and first forwardingidentifiers and second forwarding identifiers.

Two identifiers are set for the multicast packet in the embodiment ofthe present invention: the first forwarding identifier and the secondforwarding identifier. The number of the first forwarding identifiers iscontrolled according to the capacity of the switching network, and thefirst forwarding identifier may be set in various manners. In anembodiment, when the storage capacity of the switching network is verylimited, all the multicast packets with the same destination line cardhave the same first forwarding identifier, or when the storage capacityof the switching network is abundant, all the multicast packets with thesame destination line card are divided into several groups, and the samefirst forwarding identifier is set for the multicast packets in eachgroup.

An existing MID may be directly used as the second forwardingidentifier.

In Step S2, the switch fabric card receives the multicast packet, andthe first forwarding identifier and the second forwarding identifier ofthe multicast packet sent by the line card. The switch fabric cardstores the multicast packet in one or more memories.

In Step S3, the switch fabric card obtains a destination line cardcorresponding to the multicast packet according to the first forwardingidentifier, which includes at least the following two manners.

First Manner

The switch fabric card looks up a saved multicast replication tableaccording to the first forwarding identifier, and obtains thedestination line card corresponding to multicast packet, in which themulticast replication table indicates a corresponding relation betweenfirst forwarding identifiers and destination line cards.

In this manner, because a large number of data packets may correspond tothe same destination line card, the destination line card correspondingto one MID_F only needs to be saved once in the multicast replicationtable, thus significantly reducing the storage space occupied by themulticast replication table.

Second Manner

When the first forwarding identifier is set, the destination line cardcorresponding to the multicast packet is directly indicated in the firstforwarding identifier, and the switch fabric card directly obtains thedestination line card corresponding to the multicast packet according tothe first forwarding identifier.

The first forwarding identifier may be implemented by a bitmap or alink-list. For example, when the number of line cards in a router is 64,the first forwarding identifier (MID_F) may be replaced by one 64bitbitmap. When the nth bit of the bitmap is one, it means that the nthline card is a member of the multicast group. Thus, through the firstforwarding identifier implemented by the bitmap, it can be known whichline cards are the destination line cards of the current multicastpacket.

In this manner, it is not required to set a multicast replication tablefor the switch fabric card, thus reducing the requirement on the storagecapacity of the switching network to the maximum extent.

The switch fabric card extracts a multicast packet from a memory,replicates the multicast packet and then sends the multicast packet tothe destination line cards.

In Step S4, the destination line card obtains a port corresponding tothe multicast packet according to the second forwarding identifier(MID_S), and forwards the multicast packet to the port.

Each destination line card receives and stores the multicast packet sentby the switch fabric card. After the ports corresponding to themulticast packet are obtained, the multicast packet is replicated, andthen the multicast packet is sent to the ports.

The ports may be physical ports or logical ports, such as Label SwitchTunnel (LST), QinQ ports or Virtual LAN (VLAN) ports. A physical portmay contain a plurality of logical ports. If MID_S is implementedthrough an existing MID, the processing method of the Step S4 is thesame as the processing method in the prior art when a line card receivesa multicast packet and an MID.

Further, some line cards have sub-line cards. Referring to FIG. 4, theline card includes two sets of interface chips (such as a framer),forwarding chips (such as a packet processor, PP), and trafficmanagement (TM) chips, which share a switch interface, that is, the linecard has two sub-line cards.

At this time, the destination line card needs to find the destinationsub-line card corresponding to the multicast packet. In this case, inorder to ensure that the destination line card can find thecorresponding destination sub-line card, when or after the switch fabriccard obtains the destination line card corresponding to the multicastpacket and forwards the multicast packet and the MID_S to thedestination line card, the method further includes: forwarding, by theswitch fabric card, the first forwarding identifier (MID_F) to thedestination line card.

The destination line card first obtains the destination sub-line cardaccording to MID_F, and then the destination sub-line card obtains aphysical port or a logical port corresponding to the multicast packetaccording to MID_S, and forwards the multicast packet to the physicalport or logical port.

The technical solution according to the embodiment of the presentinvention sets the same first forwarding identifier for the multicastpackets with the same destination line cards by analyzing in depth thecharacteristics of the process of multicast packet forwarding and byusing the feature that many multicast packets have the same destinationline card in the first stage replication, and implements multicastpacket forwarding by using the two-stage forwarding identifiers. Thetechnical solution according to the embodiment of the present inventionremoves the redundant data stored repeatedly in the switching network,and significantly reduces the requirement on the storage capacity of theswitching network, thus implementing forwarding of a large number ofmulticast packets.

According to yet another embodiment, a network device is furtherprovided. Referring the FIG. 5, the device includes:

a receiving unit 51, configured to receive a multicast packet, and afirst forwarding identifier and a second forwarding identifier of themulticast packet;

a line card obtaining unit 52, configured to obtain a destination linecard corresponding to the multicast packet according to the firstforwarding identifier received by the receiving unit 51; and

a forwarding unit 53, configured to forward the multicast packet and thesecond forwarding identifier to the destination line card, in order thatthe destination line card obtains a port corresponding to the multicastpacket according to the second forwarding identifier.

Because in the process of two-stage replication during multicast packetforwarding, a switching network needs to know the destination line cardcorresponding to all multicast packet members of one multicast group,and the destination line card needs to know a set of physical ports orlogical ports corresponding to all multicast packet members of onemulticast group. Because the number of the line cards is one or twoorders of magnitude smaller than the number of the ports, the totalnumber of the set of the destination line cards is much smaller than thetotal number of the set of the ports.

Therefore, in the embodiment of the present invention, corresponding toeach multicast packet, two different identifiers are set for forwarding,for example, a first forwarding identifier expressed as MID_F and asecond forwarding identifier expressed as MID_S. MID_F indicates the setof the destination line cards, MID_S indicates the set of the physicalports or logical ports, and the number of MID_F is much smaller than thenumber of MID_S.

Further, referring to FIG. 6, according to the different manners ofobtaining the destination line card, the network device furtherincludes:

a saving unit 54, configured to save a multicast replication table, inwhich the multicast replication table indicates a corresponding relationbetween first forwarding identifiers and destination line cards, andmulticast packets with the same destination line card have the samefirst forwarding identifier.

At this time, the line card obtaining unit 52 is specifically configuredto look up the multicast replication table saved by the saving unitaccording to the first forwarding identifier, and obtain the destinationline card corresponding to the multicast packet.

Alternatively, when the first forwarding identifier is set, thedestination line card corresponding to the multicast packet is directlyindicated in the first forwarding identifier, and then the line cardobtaining unit 52 directly obtains the destination line cardcorresponding to the multicast packet according to the first forwardingidentifier. For example, the first forwarding identifier may beimplemented by a bitmap or a link-list. At this time, the line cardobtaining unit 52 is specifically configured to directly obtain thedestination line card corresponding to the multicast packet according tothe first forwarding identifier, in which the first forwardingidentifier is implemented by a bitmap or a link-list.

Further, by analyzing in depth the characteristics of the process ofmulticast packet forwarding and by using the feature that many multicastpackets have the same destination line card, the same first forwardingidentifier may be set for the multicast packets with the samedestination line card, so that the storage space occupied by themulticast replication table is greatly decreased, thus significantlyreducing the requirement on the storage capacity of the switchingnetwork.

The line card obtains the port corresponding to the multicast packetaccording to the second forwarding identifier, and forwards themulticast packet to the port. Further, when the line card has sub-linecards, the destination line card needs to find a destination sub-linecard corresponding to the multicast packet first. In order to ensurethat the destination line card can find the corresponding destinationsub-line card, referring to FIG. 7, the switch fabric card furtherincludes: an identifier sending unit 55, configured to forward the firstforwarding identifier to the destination line card. At this time, thedestination line card first obtains the destination sub-line cardaccording to the first forwarding identifier, and then the destinationsub-line card obtains a physical port or a logical port corresponding tothe multicast packet according to the second forwarding identifier, andforwards the multicast packet to the physical port or logical port. Atthis time, the destination line card is configured to obtain thedestination sub-line card according to the first forwarding identifier,and the destination sub-line card is configured to obtain the portcorresponding to the multicast packet according to the second forwardingidentifier and forward the multicast packet to the port.

As for the specific operation modes of functional modules and units inthe device embedment of the present invention, reference is made to themethod embodiments of the present invention. The functional modules andunits in the device embodiment of the present invention may beimplemented independently, or may be implemented by integrating in oneor more units. For example, the network device may be implemented by theswitch fabric cards.

The technical solution according to the embodiment of the presentinvention uses two identifiers to implement multicast packet forwarding.The first forwarding identifier is configured to obtain the destinationline card corresponding to the multicast packet, and the secondforwarding identifier is configured to obtain the port corresponding tothe multicast packet. By controlling the number of the first forwardingidentifiers, the capacity of the multicast replication table saved bythe switching network can be significantly reduced. The technicalsolution according to the embodiment of the present invention removesthe redundant data stored repeatedly in the switching network, andsignificantly reduces the requirement on the storage capacity of theswitching network, thus implementing forwarding of a large number ofmulticast packets.

As shown in FIG. 8, according to yet another embodiment of the presentinvention, a switching system is provided, which includes at least oneline card 82. The at least one line card includes destination linecards, that is, the at least one line card includes a destination linecard corresponding to a multicast packet. The system further includes atleast one network device 81.

The network device 81 is configured to receive a multicast packet, and afirst forwarding identifier and a second forwarding identifier of themulticast packet, obtain a destination line card 82 corresponding to themulticast packet according to the first forwarding identifier, andforward the multicast packet and the second forwarding identifier to thedestination line card 82, in order that the destination line card 82obtains a port corresponding to the multicast packet according to thesecond forwarding identifier.

The destination line card 82 is further configured to obtain the portcorresponding to the multicast packet according to the second forwardingidentifier, and forward the multicast packet to the port.

Further, the network device 81 is further configured to forward thefirst forwarding identifier to the destination line card.

The destination line card 82 includes destination sub-line cards, and isconfigured to obtain a destination sub-line card according to the firstforwarding identifier. The destination sub-line card is configured toobtain the port corresponding to the multicast packet according to thesecond forwarding identifier and forward the multicast packet to theport.

The technical solution according to the embodiment of the presentinvention uses two identifiers to implement multicast packet forwarding.The first forwarding identifier is configured to obtain the destinationline card corresponding to the multicast packet, and the secondforwarding identifier is configured to obtain the port corresponding tothe multicast packet. By controlling the number of the first forwardingidentifiers, the capacity of a multicast replication table saved by aswitching network can be significantly reduced.

The technical solution according to the embodiment of the presentinvention removes the redundant data stored repeatedly in the switchingnetwork, and significantly reduces the requirement on the storagecapacity of the switching network, thus implementing forwarding of alarge number of multicast packets.

It is clear to persons skilled in the art that the present invention maybe accomplished through software plus a necessary universal hardwareplatform. Based on this, the technical solutions of the presentinvention or the part that makes contributions to the prior art can besubstantially embodied in the form of a software product. The computersoftware product may be stored in a storage medium, such as a Read-OnlyMemory/Random Access Memory (ROM/RAM), a magnetic disk or an opticaldisk, an d contain several instructions configured to instruct acomputer device (for example, a personal computer, a server, or anetwork device) to perform the method described in the embodiments ofthe present invention or in some parts of the embodiments.

The above descriptions are merely specific embodiments of the presentinvention, but not intended to limit the protection scope of the presentinvention. Any variations or replacements that can be easily thought ofby persons skilled in the art within the technical scope of the presentinvention shall fall within the protection scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method for forwarding multicast packets,comprising: receiving a multicast packet, a first forwarding identifier,and a second forwarding identifier of the multicast packet; obtaining adestination line card corresponding to the multicast packet according tothe first forwarding identifier; and forwarding the multicast packet andthe second forwarding identifier to the destination line card so thatthe destination line card obtains a port corresponding to the multicastpacket according to the second forwarding identifier, wherein during orafter forwarding the multicast packet and the second forwardingidentifier to the destination line card, the method further comprises:forwarding the first forwarding identifier to the destination line cardso that the destination line card obtains a destination sub-line cardaccording to the first forwarding identifier; and forwarding, by thedestination sub-line card, the multicast packet to the portcorresponding to the multicast packet, wherein the port corresponding tothe multicast packet is obtained by the destination sub-line cardaccording to the second forwarding identifier, and wherein thedestination line card comprises at least two destination sub-line cards.2. The method according to claim 1, wherein obtaining the destinationline card corresponding to the multicast packet according to the firstforwarding identifier comprises: looking up a saved multicastreplication table according to the first forwarding identifier; andobtaining the destination line card corresponding to the multicastpacket, wherein the multicast replication table indicates acorresponding relation between first forwarding identifiers anddestination line cards.
 3. The method according to claim 1, whereinafter forwarding the multicast packet and the second forwardingidentifier to the destination line card, the method further comprises:obtaining, by the destination line card, the port corresponding to themulticast packet according to the second forwarding identifier; andforwarding the multicast packet to the port.
 4. The method according toclaim 1, wherein multicast packets with a same destination line cardhave a same first forwarding identifier.
 5. The method according toclaim 1, wherein obtaining the destination line card corresponding tothe multicast packet according to the first forwarding identifiercomprises directly obtaining the destination line card corresponding tothe multicast packet according to the first forwarding identifier,wherein the first forwarding identifier is implemented by a bitmap or alink-list.
 6. A network device, comprising: a receiving unit configuredto receive a multicast packet, a first forwarding identifier, and asecond forwarding identifier of the multicast packet; a line cardobtaining unit configured to obtain a destination line cardcorresponding to the multicast packet according to the first forwardingidentifier received by the receiving unit; and a forwarding unitconfigured to forward the multicast packet and the second forwardingidentifier to the destination line card so that the destination linecard obtains a port corresponding to the multicast packet according tothe second forwarding identifier; and an identifier sending unitconfigured to forward the first forwarding identifier to the destinationline card so that when the destination line card includes destinationsub-line cards, a destination sub-line card is obtained according to thefirst forwarding identifier and the multicast packet is forwarded by thedestination sub-line card to the port corresponding to the multicastpacket, wherein the port corresponding to the multicast packet isobtained by the destination sub-line card according to the secondforwarding identifier, and wherein the destination line card comprisesat least two destination sub-line cards.
 7. The network device accordingto claim 6, further comprising a saving unit configured to save amulticast replication table, wherein the multicast replication tableindicates a corresponding relation between first forwarding identifiersand destination line cards, wherein multicast packets with a samedestination line card have a same first forwarding identifier, andwherein the line card obtaining unit is configured to look up themulticast replication table saved by the saving unit according to thefirst forwarding identifier and obtain the destination line cardcorresponding to the multicast packet.
 8. The network device accordingto claim 6, wherein the line card obtaining unit is configured todirectly obtain the destination line card corresponding to the multicastpacket according to the first forwarding identifier, and wherein thefirst forwarding identifier is implemented by a bitmap or a link-list.9. A switching system, comprising: a plurality of line cards, whereinthe line cards comprise destination line cards; and at least one networkdevice, wherein the network device is configured to receive a multicastpacket, a first forwarding identifier, and a second forwardingidentifier of the multicast packet, obtain a destination line cardcorresponding to the multicast packet according to the first forwardingidentifier, and forward the multicast packet and the second forwardingidentifier to the destination line card so that the destination linecard obtains a port corresponding to the multicast packet according tothe second forwarding identifier, wherein the destination line card isconfigured to obtain the port corresponding to the multicast packetaccording to the second forwarding identifier and forward the multicastpacket to the port, wherein the network device is further configured toforward the first forwarding identifier to the destination line card,wherein the destination line card comprises destination sub-line cardsand is configured to obtain a destination sub-line card according to thefirst forwarding identifier, and wherein the destination sub-line cardis configured to obtain the port corresponding to the multicast packetaccording to the second forwarding identifier and forward the multicastpacket to the port.
 10. The system according to claim 9, whereinmulticast packets with the same destination line card have the samefirst forwarding identifier.